Voltage question

[Daniel Quinn]

If I have a 300va transformer with voltage at 30v ac.

I use a bridge rectifier to make it 30v dc.

I then use two 33,000 caps .

The measured voltage after the caps is 45v approx

Questions
Is there a way of calculating the post cap voltage?

Is 30v or 45v the voltage ?

[pre65]

According to the Hammond data sheets, the peak DC voltage of a full wave bridge rectifier followed by a capacitor is

1.41 X the secondary AC voltage.

So, 1.41 X 30 = 42.3V

The Hammond data sheet is in our STACKS section for your perusal.

https://www.audio-talk.co.uk/archive/5c007.pdf

[Daniel Quinn]

Thanks ,

For the purposes of keeping the electronics safe is the voltage 30v or 45v

Eg. I have a phono board which is maximum rated at 35v , do I need to take account of post cap voltage

[pre65]

Well, is the voltage you need to adhere to quoted as average, or peak ?

Average is 0.9 X secondary AC voltage = 27v DC

There is a power supply calculating program called PSUD 2, and it's invaluable for what you are trying to do. It's free to download. Just remember that the more accurate information you input = more accurate answer.

https://www.duncanamps.com/psud2/


*

[Nick]

Also remember that the rated voltage of 30v will be at 100% of the rated current, so in this case 10A. The regulation figure of the transformer will allow you to calculate the off load voltage. Typically it will be 5%-10%, So the expected off load voltage will be 30 * 1.414 * 1.05 (assuming 5% regulation). So 44.5v.

You will also lose 1.5v or so as the forward voltage in the rectifier. Also, if the transformer primary is rated at 230v, the actual UK voltage may be of the order of 240v, so that's another 5% or so extra to expect. So that might give you ( 1.05 * 1.414 * 30 * ( 1 + ((240 - 230) / 230 )) - 1.5 = 44.977. So close to your measured voltage.

The actual voltage under load will be a more complex calculation involving the transformer regulation and the capacitor size. Off load the cap will charge up to the peak voltage which is the 1.414, on load it will discharge every 100th of a sec to a voltage given by the cap size and DCR and the load current.

[Nick]

Well, is the voltage you need to adhere to quoted as average, or peak ?

Average is 0.9 X secondary AC voltage = 27v DC

Thats entirely wrong Phil, That looks like a calculation for a choke input supply which is a measure of the DC component of a rectified sine wave.

For the purposes of keeping the electronics safe is the voltage 30v or 45v

Eg. I have a phono board which is maximum rated at 35v , do I need to take account of post cap voltage

The voltage on the cap is the voltage the load will see, so its 45v

I would aim at a supply that produces 30v, To do that I would use a 30 / 1.414 transformer. which gives you 22v. Not a common size, 24v would give you 33.9 which with off load voltage would be above the rating you want (especially with a 300v TX feeding a phono stage), next size down is 18v, so that would give you 25v on load, maybe 26 off load with all the other bits taken into account. A phono stage is going to be such a tiny load for the supply if you use a 300va transformer that I would assume off load calculations.

[pre65]

That's what the Hammond data sheet says.

FULL WAVE BRIDGE Capacitor Input Load
V (Peak) D.C. = 1.41 X Sec. V A.C.
V (Avg) D.C. = 0.90 X Sec. V A.C.
I D.C. = 0.62 X Sec. I A.C.

[pre65]

What about keeping the 30v transformer, but using a resistive load ?

That would be 30 X .09 = 27V (or a bit more if transformer 220v rated)

[Nick]

Must be right than...

(Thats to the quote from the Hammond spec).

[pre65]

Must be right than...

(Thats to the quote from the Hammond spec).

I'm not clever enough to know.

[Nick]

What about keeping the 30v transformer, but using a resistive load ?

That would be 30 X .09 = 27V (or a bit more if transformer 220v rated)

I don't understand what that means, can you describe or draw the circuit? Maybe is the odd American way of describing the regulation of a transformer by specifying the off load voltage and then the ratio to the full load voltage. The UK does it the other way around.

[pre65]

I rely on that Hammond data sheet for rough calculations before going to PSUD 2.

If the information is incorrect, or laid out in a way that is different to normal UK workings, then I am confused.

[pre65]

Daniel, I know you like the simple approach (ie the less components the better) but had you considered choke input ?

I could give you an 0.68mH Trans Tronic choke, rated at 1A from memory.

I don't know what Nicks opinion would be for a choke loaded phono stage PSU ?

[Nick]

I think you are opening a can of worms that do not need opening. There are many advantages to a choke input supply, but I cant think of one of them that would be applicable here. And you would have to face the inherent problem of a choke input supply, that is it needing to pass a minimum current unless you don't mind it becoming a cap input supply. In those conditions when it will increase the voltage by 1.571 times. Given in this case the problem is one of over voltage that's not a good thing. Also, the fact that the load is tiny in this case removes just about all the advantages of a choke input supply.

[Daniel Quinn]

Thanks both of you

Might have known it wouldn’t be simple and straightforward. Some research me thinks. Well it’s not like I have got anything better to do